Smart Antennas for Wireless Systems Jack H. Winters May 31, 2004

Smart Antennas for Wireless
Systems
Jack H. Winters
May 31, 2004
[email protected]
12/05/03
Slide 1
Sunday, March 21, 2004
Outline
• Wireless
Impairments
• Diversity
• Combining Techniques
• Practical Issues
• Applications
Slide 2
Sunday, March 21, 2004
GOAL
In this tutorial, we will discuss current and future antenna technology for
wireless systems and the improvement that smart and adaptive antenna arrays
can provide. We will describe standard cellular antennas, smart antennas using
fixed beams, and adaptive antennas for base stations, as well as antenna
technologies for handsets and other wireless devices. We will show the
potential improvement that these antennas can provide, including range
extension, multipath diversity, interference suppression, and capacity increase.
The issues involved in incorporating these antennas into wireless systems,
including 2nd generation (CDMA, GSM, and IS-136), 3rd generation
(WCDMA and EDGE), and future cellular systems, as well as other wireless
systems, such as wireless local area networks (WLAN’s) in different
environments, such as rural, suburban, and urban areas, as well as indoors, will
be described in detail. Theoretical, computer simulation, experimental, and
field trial results will be presented. This tutorial should provide a basic
understanding of the antenna technology options and their potential in wireless
systems.
Slide 3
Sunday, March 21, 2004
WIRELESS SYSTEM IMPAIRMENTS
Wireless communication systems are limited in
performance and capacity by:
Delay
Spread
CoChannel
Interference
Rayleigh
Fading
Limited Spectrum
Slide 4
Sunday, March 21, 2004
ANTENNA DIVERSITY
Multiple antenna elements with received signals weighted and
combined
ANTENNA 1
USER
ANTENNA 2

OUTPUT
SIGNAL
ANTENNA M
With multipath, diversity gain requires independent fading:
• /4 spacing
• Direction
• Polarization
Slide 5
Sunday, March 21, 2004
ANTENNA AND DIVERSITY GAIN
Antenna Gain: Increased average output signal-to-noise ratio
- Gain of M with M antennas
- Narrower beam with /2-spaced antenna elements
Diversity Gain: Decreased required receive signal-to-noise ratio for a given BER averaged over
fading
- Depends on BER - Gain for M=2 vs. 1:
•5.2 dB at 10-2 BER
•14.7 dB at 10-4 BER
- Decreasing gain increase with increasing M - 10-2 BER:
•5.2 dB for M=2
•7.6 dB for M=4
•9.5 dB for M=
- Depends on fading correlation
• Antenna diversity gain may be smaller with RAKE receiver in CDMA
Slide 6
Sunday, March 21, 2004
DIVERSITY TYPES
Spatial: Horizontal separation
- Correlation depends on angular spread
- Only ¼ wavelength needed at terminal (10
wavelengths on base station)
Polarization: Dual polarization (doubles number of antennas in
one location)
- Low correlation
- Horizontal receive 6-10 dB lower than vertical with
vertical transmit and LOS
Slide 7
Sunday, March 21, 2004
DIVERSITY TYPES
(cont.)
Angle: Adjacent narrow beams with switched beam antenna
- Low correlation typical
- 10 dB lower signal in weaker beam, with small angular
spread
Pattern: Allows even closer than ¼ wavelength
 4 or more antennas on a PCMCIA card
 16 on a handset
 Even more on a laptop
Slide 8
Sunday, March 21, 2004
ADAPTIVE ARRAYS FOR TDMA BASE STATIONS
AT&T Wireless Services and Research - Field Trial with Lucent
7/96-10/96
24 (12 ft)
3 (1.5 ft)
Field trial results for 4 receive antennas on the uplink:
3 (1.5 ft)
• Range extension: 40% reduction in the number of base stations can be obtained 4 to 5 dB
greater margin  30% greater range
• Interference suppression: potential to more than double capacity
Operation with S/I close to 0 dB at high speeds  greater capacity and quality
Slide 9
Sunday, March 21, 2004
Diversity Antennas
Base Station Antennas
• Antennas mounted on 60 foot tower on 5 story office
building
• Dual-polarized slant 45 1900 MHz sector antennas and
fixed multibeam antenna with 4 - 30 beams
Laptop Prototype
• 4 patch antennas at 1900 MHz separated
by 3 inches (/2 wavelengths)
• Laptop prototype made of brass with
adjustable PCB lid
Slide 10
Sunday, March 21, 2004
COMBINING TECHNIQUES
Selection:
Output
• Select antenna with the highest received signal power
• P0M = P0M
Slide 11
Sunday, March 21, 2004
COMBINING TECHNIQUES (CONT.)
Maximal ratio combining:
W1

Output
WM
• Weight and combine signals to maximize signal-to-noise ratio (Weights
are complex conjugate of the channel transfer characteristic)
• Optimum technique with noise only
• BERM  BERM (M-fold diversity gain)
Slide 12
Sunday, March 21, 2004
OPTIMUM COMBINING (ADAPTIVE
ANTENNAS)
• Weight and combine signals to maximize signal-tointerference-plus-noise ratio (SINR)
- Usually minimize mean squared error (MMSE)
• Utilizes correlation of interference at the antennas to
reduce interference power
• Same as maximal ratio combining when interference is
not present
Slide 13
Sunday, March 21, 2004
INTERFERENCE NULLING
Line-Of-Sight Systems
User 1

•
•
•
User 1
Signal
User 2
Utilizes spatial dimension of radio environment to:
• Maximize signal-to-interference-plus-noise ratio
• Increase gain towards desired signal
• Null interference: M-1 interferers with M antennas
Slide 14
Sunday, March 21, 2004
INTERFERENCE NULLING
Multipath Systems
User 1

•
•
•
User 1
Signal
User 2
Antenna pattern is meaningless, but performance is based on the number of
signals, not number of paths (without delay spread).
=> A receiver using adaptive array combining with M antennas and N-1 interferers can
have the same performance as a receiver with M-N+1 antennas and no interference, i.e.,
can null N-1 interferers with M-N+1 diversity improvement (N-fold capacity increase).
Slide 15
Sunday, March 21, 2004
Multiple-Input Multiple-Output (MIMO) Radio
•
•
With M transmit and M receive antennas, can provide M independent channels, to increase data rate Mfold with no increase in total transmit power (with sufficient multipath) – only an increase in DSP
–
Indoors – up to 150-fold increase in theory
–
Outdoors – 8-12-fold increase typical
Measurements (e.g., AT&T) show 4x data rate & capacity increase in all mobile & indoor/outdoor
environments (4 Tx and 4 Rx antennas)
–
216 Mbps 802.11a (4X 54 Mbps)
–
1.5 Mbps EDGE
–
19 Mbps WCDMA
Slide 16
Sunday, March 21, 2004
Slide 17
Sunday, March 21, 2004
Practical Issues
Interferers
• # interferers  M
But:
• Only need to suppress interference into the noise (not eliminate)
• Usually only 1 or 2 dominant interferers
Result:
• Substantial increase in performance and capacity even with a few (even 2)
antennas
Note:
• Optimum combining yields interference suppression under all conditions
(e.g., line-of-sight, Rician fading)
Slide 18
Sunday, March 21, 2004
Delay Spread
Channel Model D – 802.11n
30
25
Relative dB
20
15
dB
10
5
0
-5 0
0
50
1 00
15 0
200
25 0
300
3 50
40 0
D e la y in N an o s e c on ds
Figure 1. Model D delay profile with cluster extension (overlapping clusters).
Slide 19
Sunday, March 21, 2004
EQUALIZATION
• Delay spread: Delay spread over [(M-1) / 2]T or M-1 delayed signals
(over any delay) can be eliminated
• Typically use temporal processing with spatial processing for
equalization:
LE

MLSE/DFE
LE
• Spatial
processing followed by temporal processing has degradation,
but this degradation can be small in many cases
Slide 20
Sunday, March 21, 2004
CORRELATION
• Degradation due to fading correlation with adaptive array that combats
fading, suppresses interference, and equalizes delay spread is only slightly
larger than that for combating fading alone:
- Small degradation with correlation less than 0.5
BER with Correlation
Model
10-1
User 1

BER
D/=0.382

3.82
=18dB
10-2
0.382

3.82
=27dB
D
1
D
2
M-1
M
10-3
180
Slide 21
100 90
50
 (Degrees)
0
Sunday, March 21, 2004
SMART ANTENNAS
Today: Cellular systems with sectorization (120°) 
handoffs between sectors
f4
f3
f1
f5
f6
f2
For higher performance  Narrower sectors  Too many handoffs
Smart Antenna: Multibeam antenna or adaptive array without
handoffs between beams
Slide 22
Sunday, March 21, 2004
Smart Antennas
Adaptive Antenna Array
Switched Multibeam Antenna
SIGNAL
BEAMFORMER
SIGNAL
BEAM
SELECT
SIGNAL
OUTPUT
SIGNAL
OUTPUT
INTERFERENCE
INTERFERENCE
BEAMFORMER
WEIGHTS
Smart antenna is a multibeam or adaptive antenna array that tracks the wireless environment to
significantly improve the performance of wireless systems. Multibeam less complex, but applicable
mainly outdoors, while:
Adaptive arrays in any environment provide:
• Antenna gain of M
• Suppression of M-1 interferers
In a multipath environment, they also provide:
• M-fold multipath diversity gain
• With M Tx antennas (MIMO), M-fold data rate increase in same channel with same total transmit power
Slide 23
Sunday, March 21, 2004
Multibeam Antenna (Phased Array)
• Fixed (or steerable) beams
• Consider cylindrical array with M elements (/2 spacing)
- Diameter  (M / 4) feet at 2 GHz
r
•With small scattering angle ( = 4):
Mobile
- Margin = 10log10M (dB)

- Number of base stations = M-1/2
- Range = M1/4
• Disadvantages:
Base Station
- No diversity gain (unless use separate antenna)
- With large scattering angle , gain is limited for beamwidths  
Slide 24
Sunday, March 21, 2004
Range Increase for IS-136
Fixed Multibeam Antenna
• Increases gain for better coverage
• Range increase is limited by
angular spread
• No spatial diversity gain
• Can be used on downlink or uplink
Adaptive Array
• Range increase independent of
angular spread
• Diversity gain increases with
antenna spacing
• Can be used on uplink with fixed
multibeam downlink
Slide 25
Sunday, March 21, 2004
Range Increase with CDMA Signals
Single beam for all
RAKE fingers results in
range limitation with
angular spread for
multibeam antenna
(phased array)
Slide 26
Sunday, March 21, 2004
Range Increase with CDMA Signals - Different
Beams per Finger
7
6
Adaptive Array
3M-fold
Diversity
Phased Array
Normalized
Range
Theory
60°
45°
20°
3-fold
10°
3° Diversity
0=3°
10°
20°
45°
60°
5
4
3
5 Spacing
FIXED SECTORS, 0=60°
2
10
1
2
3
log10 (M)
Slide 27
Sunday, March 21, 2004
WEIGHT GENERATION TECHNIQUES
For Smart Antenna: Need to identify desired signal and
distinguish it from interference

•
•
•
Weight
Generation
Blind (no demod): MRC – Maximize output power
Interference suppression – CMA, power inversion, power
out-of-band
Non-Blind (demod): Training sequence/decision directed reference signal
MIMO needs non-blind, with additional sequences
Slide 28
Sunday, March 21, 2004
Can be implemented Analog (RF) or Digital
• Analog Advantages:
• Digital requires M complete RF chains, including M A/D and D/A's,
versus 1 A/D and D/A for analog, plus substantial digital signal
processing
• The cost is much higher for digital
• An appliqué approach is possible - digital requires a complete
baseband
• Digital Advantages:
• Slightly higher gain in Rayleigh fading (as more accurate weights
can be generated)
• Temporal processing can be added to each antenna branch much
easier than with analog, for higher gain with delay spread
• Modification for MIMO (802.11n) possible
Slide 29
Sunday, March 21, 2004
Appliqué
- Cellular – IS-136
- WLANs – 802.11a/b/g
- WiMAX – 802.16
Wireless
Transceiver
Baseband/MAC
RF
Appliqué
(Spatial
processing
only)
RF
Processor
Processor
(including
temporal
equalization),
Host Interface
Slide 30
Sunday, March 21, 2004
APPLICATIONS
• Chronology
• Cellular
• Mobile Satellite
• WLAN
Slide 31
Sunday, March 21, 2004
SMART/ADAPTIVE ANTENNA
ARRAY TECHNOLOGY
Commercial
•mobile, indoor, wireless local loop
•range extension
•interference reduction with fast fading
•signal acquisition and tracking
•delay spread equalization
•propagation characterization
•adaptive retransmission
•antenna design and implementations
Military
•high resolution direction-finding
• jammer cancellation
•interference reduction
•signal classification
•directional transmission
•custom VLSI implementations
Research
Applications
1980
•long range surveillance radars
•military communication systems
•sonar
•geophysical exploration
•imaging
1990
•Nortel SmartBTS - GSM
•MetaWave SpotLight
•ArrayComm IntelliCell
•Celwave Smart System - AMPS
•Hazeltine IAS - AMPS
•Ericsson and Lucent - IS-136
Slide 32
2000
• 3G
• WLAN
• WiMAX
• UWB
• 802.20
• Satellite radio/TV
Sunday, March 21, 2004
Smart Antennas for IS-136
• Key enhancement technique to increase system capacity, extend coverage, and
improve user experience in cellular (IS-136)
SIGNAL
Uplink Adaptive Antenna
SIGNAL
OUTPUT
INTERFERENCE
BEAMFORMER
WEIGHTS
SIGNAL
In 1999, combining at TDMA base stations changed
from MRC to MMSE for capacity increase
Slide 33
BEAMFORMER
Downlink Switched Beam Antenna
BEAM
SELECT
SIGNAL
OUTPUT
Sunday, March 21, 2004
IS-136
• TDMA with 3 users per channel
• /4 DQPSK at 48.6 kbps
• 162 symbols/slot
• 14 symbol synchronization sequence
• Two receive antennas at base (Tracking over slot, but spatial processing before equalization is adequate)
IS-136 Timing Structure
Digital Traffic Channel
TDMA FRAME 40 ms
1
2
3
4
5
6
TIME SLOT 6.687 ms (162 symbols)
3
3
8
14
61
6
6
61
G
R
DATA
SYNC
DATA
SACCH
CDVCC
DATA
MOBILE TO BASE
14
SYNC
6
SACCH
65
DATA
6
CDVCC
65
DATA
1
RSVD
5.5
CDL
BASE TO MOBILE
Symbol duration 41 s (48.6 kb/s)
Slide 34
Sunday, March 21, 2004
GSM
• TDMA with 8 users per channel
• Gaussian MSK at 270.833 kbps
• 156.25 bits/slot
• 26 bit synchronization sequence
• Two receive antennas at base (weights fixed over slot, but
S-T processing is needed)
Frame
4.615 ms
1
2
3
4
5
6
7
8
T
Data
F
Train
F
Data
T
Guard
3
57 b
1
26
1
57
3
8.25
Key:
T: Tail Bit
F: Flag
Train: Equalizer Training Sequence
577 s
Slot
Slide 35
Sunday, March 21, 2004
SMART ANTENNAS IN THIRD
GENERATION SYSTEMS: EDGE
• High data rate ( 384 kbps) service based on GSM, for both Europe
and North America
• 8PSK at 270.833 ksps
• 26 symbol training sequence
• 1/3, 3/9 or 4/12 reuse
3
58
26
58
3 8.25
576.92 s
Slide 36
Sunday, March 21, 2004
ADAPTIVE ARRAYS IN EDGE
Spatial-Temporal processing using DDFSE for interference suppression
Issues: tracking, dual antenna terminals
Slide 37
Sunday, March 21, 2004
Example of LOS Case
with No Diversity:
Ultralow Profile Mobile
Satellite
Slide 38
Sunday, March 21, 2004
Mobile DBS Limitation
Legacy Products Too Large and Bulky for Minivan/SUV Market
Slide 39
Sunday, March 21, 2004
Hybrid Beam Steering Approach
•
•
Electronic Beam Steering in Elevation Direction
Mechanical Beam Steering in Azimuth Direction
– Most Cost Effective Approach
– Achieve the Lowest Profile
Slide 40
Sunday, March 21, 2004
Structure of the Phased Array Antenna
LHCP
Beam
Radiation slots
LHCP output
RHCP
Beam
RHCP output
• Two CPs are Generated by Direction of Wave Traveling
within the Waveguide
Slide 41
Sunday, March 21, 2004
OEM
Aftermarket
Incorporation into vehicles
Slide 42
Sunday, March 21, 2004
Wireless System Enhancements
Peak Data Rate
UWB
100 Mbps
3.1-10.6 GHz
High performance/price
WiMAX
802.11a/g
2.4, 5.5GHz Unlicensed
10 Mbps
802.11b
2.4GHz Unlicensed
1 Mbps
$/Cell
$/Sub
$ 500,000
$ 500
$ 1000
$ 100
$ 100
$ 10
Enhanced
BlueTooth
100 kbps
2.4GHz
High ubiquity and mobility
2G/3G Wireless
0.9, 2GHz
10 feet
2 mph
100 feet
1 mile
10 mph
30 mph
Slide 43
10 miles
60 mph
Range
Mobile Speed
Sunday, March 21, 2004
Smart Antennas for WLANs
Smart
Antenna
AP
Smart
Antenna
Smart
Antenna
AP
Interference
Smart Antennas can significantly improve the performance of WLANs
• TDD operation (only need smart antenna at access point or terminal for performance
improvement in both directions)
• Higher antenna gain  Extend range/ Increase data rate/ Extend battery life
• Multipath diversity gain  Improve reliability
• Interference suppression  Improve system capacity and throughput
– Supports aggressive frequency re-use for higher spectrum efficiency, robustness in the ISM
band (microwave ovens, outdoor lights)
– Data rate increase  M-fold increase in data rate with M Tx and M Rx antennas (MIMO
802.11n)
Slide 44
Sunday, March 21, 2004
Appliqué
- Cellular – IS-136
- WLANs – 802.11a/b/g
- WiMAX – 802.16
Wireless
Transceiver
Baseband/MAC
RF
Appliqué
(Spatial
processing
only)
RF
Processor
Processor
(including
temporal
equalization),
Host Interface
Slide 45
Sunday, March 21, 2004
Smart Antenna WiFi (PCMCIA Reference Design)
PCMCIA - CARDBUS Interface
Appliqué Architecture Plug-and-Play to legacy designs
Legacy Transceiver
Baseband/MAC
RF
Processor
Processor
Motia
Smart Antenna
RF Chip
Partners: Intersil/Globespan,
Maxim/TI, RFMD, Atmel
Slide 46
Sunday, March 21, 2004
802.11b Performance with Fading
4-antennas (baseline) achieves a 12 to 14 dB gain
over a single antenna
Performance Comparison - All four data rate
0.8
0.7
802.11 spec
11Mbps Baseline
2Mbps Baseline
5.5Mbps Baseline
1Mbps Baseline
11Mbps 1-ant
5.5Mbps 1-ant
2Mbps 1-ant
1Mbps 1-ant
Poly. (1Mbps Baseline)
Poly. (2Mbps Baseline)
Poly. (5.5Mbps Baseline)
Poly. (11Mbps Baseline)
Expon. (11Mbps 1-ant)
Expon. (5.5Mbps 1-ant)
Expon. (2Mbps 1-ant)
Expon. (1Mbps 1-ant)
0.6
FER
0.5
0.4
0.3
0.2
0.1
y = 4.1054e-0.1845x
0
-10
-5
0
5
Theoretical for short packet
Slide 47
10
15
20
25
SNR (dB)
Sunday, March 21, 2004
30
802.11b Beamforming Gains with 4 Antennas
Performance Gain over a Single Antenna in a
Rayleigh Fading Channel
2 Antenna
Selection
Adaptive
Adaptive
One Side
Both Sides
Theoretical Bound
Both Sides
6.1 dB
12.8 dB
18.0 dB
22.2 dB
2X to 3X Range +
Uniform Coverage
3X to 4X Range +
Uniform Coverage
Slide 48
Sunday, March 21, 2004
802.11a/g
Flat Rayleigh Fading
24Mbps, Short Packet
1
1 Ant
2 Ant, Selective
8 symbols/packet
4 Ant, Selective
RF Beamforming
4 Ant, Motia
RF Beamforming
2 Ant, Motia
BB Beamforming
BB Beamforming
2 Ant, Motia
BB Beamforming
w/ Ideal
Weight
BB Beamforming
w/Ideal
Weight
4 Ant, Motia
BB Beamforming
BB Beamforming
PER
4 Ant, Motia
BB Beamforming
w/ Ideal
Weight
BB Beamforming
w/Ideal
Weight
0.1
0.01
5
7
9
11
13
15
17
19
21
23
25
27
29
31
SNR (dB)
Slide 49
Sunday, March 21, 2004
802.11a/g
50ns Exp Decay Rayleigh Fading
24Mbps, Short Packet
1
PER
1 Ant
2 Ant, Selective
4 Ant, Selective
RF Beamforming
4 Ant, Motia
RF Beamforming
BB
Beamforming
2 Ant, Motia BB Beamforming
BB Beamforming
w/Ideal
Weight
2 Ant, Motia
BB Beamforming
w/ Ideal
Weight
4 Ant, Motia
BB
Beamforming
BB Beamforming
BB Beamforming
w/Ideal
Weight
4 Ant, Motia
BB Beamforming
w/ Ideal
Weight
8 symbols/packet
0.1
0.01
7
9
11
13
15
17
19
21
23
25
27
SNR (dB)
Slide 50
Sunday, March 21, 2004
802.11a/g Beamforming Performance Summary
Beamforming Gain (dB) @ 10% PER
6 Mbps
24 Mbps
54 Mbps
Summary
Short Packet
Long Packet
Short
Packet
Flat Rayleigh Fading
11
11
12
12
12
12
11 ~ 12
50ns Exp Decay Rayleigh
Fading
8
10
7
7
8
9
7 ~ 10
100ns Exp Decay Rayleigh
Fading
6
6
5
5
6
7
5~7
200ns Exp Decay Rayleigh
Fading
4
9
5
6
Very High Error
Floor
Long Packet
Short Packet
Long Packet
4~9
Very High
Error Floor
Slide 51
Sunday, March 21, 2004
802.11n
•
•
Requirements for 802.11n:
–
>100 Mbps in MAC
–
>3 bits/sec/Hz
–
Backward compatible with all 802.11 standards
Requires MAC changes and may require MIMO:
–
•
4X4 system (?)
Next standards meeting in Portland
Slide 52
Sunday, March 21, 2004
Smart Antennas for Wireless Systems
Conclusions
•
•
•
Smart antennas can improve user experience and system capacity by reducing
interference, extending range, increasing data rates, and improving quality
Smart antennas can be implemented in the physical layer with little or no impact on
standards
Expertise and experience in the development and deployment of smart antennas for
cellular can be applied to develop smart antennas for WLANs, and many other
wireless applications
Slide 53
Sunday, March 21, 2004